Pressure lines (e.g., pressure vessels and piping systems) are often designed with pressure relief valves located at various locations to protect the pressure line from excess overpressure. The pressure relief valves are self-actuated devices set to open when the pressure in the pressure line exceeds a specified level. When the pressure in the pressure line exceeds the pressure at which the pressure relief valve is set to open, the closure element of the pressure relief valve moves away from the inlet nozzle seat and fluid is allowed to flow out of the pressure line and through the pressure relief valve. This flow of fluid will continue at a sufficient rate to prevent the pressure in the pressure line from rising above a predetermined level or above a specified overpressure. When the pressure in the pressure line is reduced to a level below the pressure at which the pressure relief valve is set to open, the closure element in the pressure relief valve will return to its closed position, i.e. into contact with the inlet nozzle seat, preventing additional flow from the pressure line. Under normal operating conditions, the closure element of the pressure relief valve is in the closed position.
Prior art monitoring devices used in these pressure lines typically employ position transducers mounted on the pressure relief valve to sense the position of the closure element. These position transducers transmit analog signals indicating the position of the closure element with respect to the inlet nozzle. These devices, however, do not store this information and apply the information to determine operating characteristics of the pressure relief valve, such as total flow through the pressure relief valve during a specified time interval when the pressure relief valve is open.
Moreover, in the prior art the presence of leakage flow past the closure element of a pressure relief valve (i.e., flow past the closure element when the closure element is in the closed position) could only be determined by physically examining the valve in its installed position, removing the valve from its installed position, and performing a seat leakage test on a test stand, or by isolating the valve (through the use of appropriate valving) in its installed position, but not in active service, and performing a seat leakage test in situ. Such techniques for determining the presence of seat leakage, however, do not allow for continuous monitoring to detect seat leakage past the closure element while the pressure relief valve is both installed and in service.
In addition, unstable operation of pressure relief valves, i.e. rapid opening and closing of the closure element, can occur when the system pressure rises just to or slightly above the set pressure and then drops, as a result of fluid flowing from the system through the pressure relief valve, as soon as the closure element lifts off the seat permitting the spring to immediately seat the closure element. Such unstable operation, however, can cause physical damage to components of the pressure relief valve. It is therefore desirable to know when such unstable operation occurs so that corrective action may be taken. The prior art practice has been for personnel to listen for the noise, often referred to as "valve chatter," generated by the closure element being rapidly and repeatedly forced against its seat. This practice, however, is ineffective if no personnel are near the valve at the time the unstable operation occurs or if the location of the valve is beyond earshot of attending personnel.